(1) Field of the Invention
The present invention relates to a power transmission gearbox, and also to an aircraft provided with such a gearbox.
In particular, an aircraft of the rotorcraft type has a rotor that contributes to providing the aircraft with at least part of its lift. A rotorcraft of the helicopter type may thus have a “main” rotor for providing it with lift and with propulsion.
(2) Description of Related Art
In order to set such a rotor into rotation, a rotorcraft is provided with a power plant including at least one engine.
In addition, a gearbox may be interposed between the engines and a rotor in order to transmit the power developed by the engines to the rotor. A particular function of the gearbox is to drive the rotor at a speed of rotation that is low compared with the high speed of rotation of the engines. By way of illustration, a turboshaft type engine has a drive shaft that rotates at a speed of the order of several tens of thousands of revolutions per minute, while the rotor rotates at a speed of only about 300 revolutions per minute.
Thus, a helicopter is provided with at least one engine that drives at least one main rotor via a main gearbox.
A prior art gearbox presents one mechanical drive system for each engine. Each inlet mechanical drive system is thus driven by a respective engine.
Furthermore, the inlet mechanical drive system acts together to drive a wheel, sometimes referred to as the “big” wheel.
The big wheel drives at least one speed reduction stage in order to drive rotation of the rotor via gearing.
The sets of gears present in helicopter main gearboxes need to be lubricated permanently, possibly in degraded manner as a result of certain failures.
Under such circumstances, a gearbox has a main lubrication system. Such a main lubrication system has a lubrication pump. For example, the big wheel may be constrained to rotate with a wheel that drives the lubrication pump.
The lubrication pump then transfers a lubrication liquid from a bottom of the gearbox to main spray means. The main spray means may comprise nozzles, for example.
The lubrication liquid drawn from the bottom of the gearbox is usually hot. Consequently, a lubrication system may include a heat exchanger arranged downstream from the main spray means in order to cool the lubrication liquid.
An emergency lubrication pump is sometimes arranged in the bottom of the gearbox in order to feed the main spray means, possibly without passing via the heat exchanger. The emergency lubrication pump operates continuously or it is put into operation automatically following a failure of the pump of the main lubrication system, for example.
In the event of a leak of the cooling liquid, regulations in force also require the gearbox to remain mechanically sound for a certain minimum duration in order to enable a crew to attempt to land the aircraft as quickly as possible.
Certain present technical solutions thus consist in providing an additional lubrication system in order to satisfy this requirement. Such an additional system may comprise an additional tank of lubrication liquid and an additional pump enabling the lubrication liquid to be caused to flow in the ducts provided with spray means that are dedicated to this additional system.
Such an additional system is found to be effective, but it presents numerous additional pieces of equipment, not only a pump, but also a tank and spray means. The additional system may be heavy and difficult to arrange in an environment that is encumbered by numerous members.
In this context, EP 0 540 406 describes an additional cooling system of this type.
The additional system includes an inlet orifice and an outlet orifice, which are arranged in a gearbox casing. The inlet orifice and the outlet orifice are opened under the action of a control device as from a predetermined value of a parameter that is representative of a loss of lubrication oil.
An air pipe leads to the inlet orifice in order to convey air. The air pipe may run from an air intake of the aircraft or from a compressor of an engine.
In addition, a supply of lubricant external to the gearbox is provided in order to lead into the air pipe. The supply of lubricant is thus connected to a feed tube that is arranged substantially at the center of the air pipe.
The supply of lubricant is arranged under the air pipe.
Document EP 0 448 690 describes an emergency lubrication system for use in the event of a failure of a primary lubrication system.
That emergency lubrication system comprises an oil tank that is filled from the primary lubrication system. Furthermore, the emergency lubrication system has a dedicated oil sprayer and a source of air that is external to the gearbox. The source of air may comprise air bled from an engine or indeed an auxiliary air compressor.
Document U.S. Pat. No. 4,717,000 describes an emergency lubrication system. That emergency lubrication system has an emergency tank that is normally fed from a primary lubrication system.
The emergency lubrication system includes a source of compressed air in the form of a compressor of an engine external to the gearbox, a valve that is sensitive to oil pressure, and mist generators dedicated to the emergency lubrication system.
Documents EP 0 443 901, US 2007/261922, U.S. Pat. No. 5,046,306, FR 2 826 094, U.S. Pat. No. 4,976,335, US 2014/090930, and WO 2011/059450 are also known.